In OFDM communications systems the frequencies and modulation of a frequency-division multiplexing (FDM) communications signal are arranged orthogonal with each other to eliminate interference between signals on each frequency. In this system, low-rate modulations with relatively long symbols compared to the channel time characteristics are less sensitive to multipath propagation issues. OFDM thus transmits a number of low symbol-rate data streams on separate narrow frequency subbands using multiple frequencies simultaneously instead of transmitting a single, high symbol-rate data stream on one wide frequency band on a single frequency. These multiple subbands have the advantage that the channel propagation effects are generally more constant over a given subband than over the entire channel as a whole. A classical In-phase/Quadrature (I/Q) modulation can be transmitted over individual subbands. Also, OFDM is typically used in conjunction with a Forward Error Correction scheme, which in this instance, is sometimes termed Coded Orthogonal FDM or COFDM.
As known to those skilled in the art, an OFDM signal can be considered the sum of a number of orthogonal subcarrier signals, with baseband data on each individual subcarrier independently modulated, for example, by Quadrature Amplitude Modulation (QAM) or Phase-Shift Keying (PSK). This baseband signal can also modulate a main RF carrier.
These types of multi-carrier waveforms for digital communications require a summation of multiple frequency-spaced single-carriers prior to transmission through a Power Amplifier (PA). OFDM systems typically use a hardware efficient IFFT to modulate each individual subcarrier with a QAM symbol and sum the modulated complex exponentials together to produce a single time-domain waveform, which has a very large Peak-to-Average Power (PAPR) ratio. As a result, the average power into the PA must be “backed-off” to avoid clipping of the time-domain signal peaks. This clipping significantly increases the in-band noise (IBN) and the out-of-band noise (OBN) and adversely increases the Bit Error Rate (BER) and the Adjacent Channel Interference (ACI), respectively.
Terrestrial wireless communications systems usually encounter multipath fading channels, which are typically modeled using a Rician direct path and several additional Rayleigh paths. Most standard OFDM systems use a set of known training symbols to estimate the wireless channel's frequency response. These training symbols are transmitted during the beginning of a packet and form the preamble. Occasionally training symbols are transmitted during the middle of a packet and form a mid-amble. This preamble provides to the receiver known amplitude and phase references at each of the subcarrier frequencies. The preamble X(f) is stored in memory and known in advance at the receiver. The transmitter sends x(t) [the inverse Fourier Transform of X(f)] over the wireless channel to the receiver. During its transmission to the receiver, the transmitted preamble representation x(t) is convolved with the channel's time response h(t) to produce y(t) at the receiver. The receiver uses Y(f) [the Fourier transform of y(t)] to calculate H(f)=Y(f)/X(f), the channel frequency response. In order to compensate for the channel response at the receiver, incoming data symbols are multiplied by H−1(f), which is equivalent to the convolution in the time-domain with the inverse channel response.
Conventional OFDM systems use a preamble designed for a specific number of subcarriers. Each preamble has a low PAPR by design. Optimizing a preamble's PAPR for a certain number of subcarriers N does not hold when the number of subcarriers changes. Therefore, if the number of subcarriers in the system changes, a new low PAPR preamble based on the changed (but fixed) number of subcarriers is required. Thus, current OFDM systems design and implement special preambles for each fixed number of subcarriers. Typically, OFDM implementations that allow a different number of subcarriers have a small number of combinations of subcarrier sizes and preambles to select from based on which fixed number of subcarriers will be transmitted. In practical OFDM systems, the PAPR may be reduced using one or a combination of several techniques.
Some OFDM systems use nonlinear signal distortion such as hard clipping, soft clipping, companding, or predistortion techniques. These nonlinear distortion techniques are implemented in fairly simple circuits. They do not work well, however, in cases where the OFDM subcarriers are modulated with higher order modulation schemes. In such situations, the Euclidian distance between the symbols is relatively small and the additional noise introduced by the PAPR reduction causes significant performance degradation.
A second group of OFDM systems reduce PAPR using various coding techniques, which are typically distortionless. The PAPR reduction is most commonly achieved by eliminating symbols having a large PAPR. To obtain an appreciable level of PAPR reduction, however, high redundancy codes are used and as a result, the overall transmission efficiency is reduced.
A third group of OFDM systems minimize PAPR based on OFDM symbol scrambling and selecting a sequence that produces minimum PAPR. These pre-scrambling techniques achieve good PAPR reduction, but typically require multiple FFT transforms and higher processing power. An example of OFDM systems using some type of scrambling are the OFDM communications systems and methods disclosed in commonly assigned U.S. patent application Ser. Nos. 11/464,877; 11/464,857; 11/464,854; 11/464,861; and 11/464,868, filed on Aug. 16, 2006, the disclosures which are hereby incorporated by reference in their entirety. Further enhancements to those systems are found in commonly assigned U.S. patent application Ser. Nos. 12/060,283; 12/060,311; and 12/060,292, filed Apr. 1, 2008, the disclosures which are hereby incorporated by reference in their entirety.
An OFDM waveform has a very large Peak to Average Power Ratio (PAPR). To avoid clipping and nonlinear distortion, the OFDM transmitter's Power Amplifier (PA) needs to be operated a significantly lower power level than its peak rating (i.e., “backed off”). This paper proposes a novel low PAPR preamble for channel estimation. This solution yields a preamble with a typical PAPR of 2.6 dB, regardless of the number of subcarriers. The subcarriers are evenly spaced and equal amplitude, ideal for channel sounding. The result is a preamble construction that operates over a variable number of subcarriers, improves SNR, and is easy to implement. The theoretical basis for the technique is presented, and a particular implementation of the technique in hardware is discussed. Both simulations and measurements demonstrate very significant benefits of the technique in long-range wireless communications applications that use an OFDM waveform for radio communications.